Breathing freely threatens seeing clearly.

People with asthma breathed a sigh of relief with the advent of inhaled steroids in the early 1990s. These widely used, synthetic hormones relieve inflammation where it counts-in the lungs and bronchial passages-without the severe side effects of oral steroids.

So effective are inhaled steroids at staving off asthma attacks that doctors now regard them as a mainstay of therapy. Thousands of people take high doses daily in regimens that may last a lifetime.

New research indicates, however, that inhaled steroids may have an unsuspected dark side.

A study of nearly 50,000 elderly people in the Canadian province of Quebec found that prolonged, high-dose use markedly increases a person's risk of glaucoma, the leading cause of blindness.

"This was a surprise," reports Samy Suissa of McGill University Health Centre in Montreal. "We've been told inhaled steroids are safe because they act on the lungs and are not diffused through the body."

Before inhaled steroids arrived in drug stores, doctors often prescribed potent oral versions of the drug, which have severe drawbacks. Among other things, they can suppress the adrenal gland and throw the body's hormone balance out of whack. In older people, oral versions cause osteoporosis, which weakens bones.

Many studies of inhaled steroids found them to be relatively free of such side effects. Researchers attribute this safety profile to the body's ability to clear most inhaled steroids from the bloodstream.

Eighty percent of each inhalation collects in the mouth and throat; swallowing carries it to the liver, where it is metabolized. About 20 percent of each dose enters the lungs. Oxygen ushered from the lungs into the blood carries some of the inhaled asthma medicine along with it. "Steroids are apt to be disseminated throughout the body," Suissa asserts. "We wondered whether taking steroids directly into the lungs would carry them into the eyes."

Suissa and his colleagues began their study after seeing reports of two cases of glaucoma in people using inhaled steroids. Other researchers had previously reported that steroid eye drops can cause the disease. Although no one understands precisely why glaucoma occurs, doctors have learned how the disease progresses. The eye fills with excess fluid, which puts pressure on the optic nerve, the lens, and other structures, dislodging them.

The McGill team looked for an association between inhaled steroids and glaucoma in people age 66 and older whose records appear in Quebec's centralized medical records database.

Between 1988 and 1994, 9,793 people in this group had been diagnosed as having glaucoma. For comparison, the researchers selected at random 38,325 people from the same database who did not have the disease. They found that those who had used high doses of inhaled corticosteroids for at least 3 months were 44 percent more likely to have developed glaucoma. The researchers report their findings in the March 5 Journal of the American Medical Association.

In the United States, where about 100,000 older people use high-dose inhaled steroids, this elevated risk translates into an estimated one-third of the 3,000 cases of glaucoma among people over 65, Suissa says.

"It's an interesting finding," says Lea Davies of Georgetown University Medical Center in Washington, D.C. She adds that it would be interesting to find out whether inhaled steroids pose a similar risk to children.

Suissa, whose study did not address this issue, agrees that young people, who could take inhaled steroids for decades, may face an increased risk.

He cautioned against abandoning inhaled steroids, however, because they remain the best way to prevent sometimes fatal asthma attacks. People who use the inhalers should talk to their doctor about the risk of glaucoma and get their eyes checked at regular intervals, he advises.